1. Field of the Invention
This invention relates to an image forming apparatus and an electric-power control method, and more particularly, to an image forming apparatus that uses a ceramic surface heating system as a heating means and to an electric-power control method.
2. Description of the Related Art
Conventionally, an image forming apparatus that uses an electrophotographic process is known. A thermal fixing device in the image forming apparatus fixes an unfixed image (toner image) formed on transfer paper by an image forming means, such as an electrophotographic process, onto the transfer paper. A device having a roller heating system that uses a halogen heater as a heat source or a device having a film heating system that uses a ceramic surface heater as a heat source is known as the thermal fixing device. For example, see Japanese Patent Application Laid-open Nos. 63-313182 (1988), 2-157878 (1990), 4-044075 (1992), 4-044076(1992), 4-044077(1992), 4-40078(1992), 4-044079 (1992), 4-044080(1992), 4-044081 (1992), 4-044082 (1992), 4-044083(1992), 4-204980(1992), 4-204981(1992), 4-204982 (1992), 4-204983 (1992), and 4-204984 (1992).
A heater is connected to an AC power source through a switching element such as a triac. Electric power is supplied from the AC power source to the heater. A thermal fixing device that uses the heater as a heat source includes a temperature detecting element, e.g., a thermistor temperature-sensitive element. The temperature detecting element detects the temperature of the thermal fixing device and transmits detected temperature information to a sequence controller. The sequence controller performs temperature control so that the switching element is turned on or of f based on the detected temperature information, and thereby a power supply to the heater, which is the heat source of the thermal fixing device, is turned on or off so that the thermal fixing device can reach a desired temperature. A ceramic surface heater is turned on or off by controlling the phase or wavenumber of the AC power source.
When the temperature of the thermal fixing device is controllably adjusted, the sequence controller calculates a difference in the electric power supplied to the heater by a comparison between the temperature detected by the thermal detector and a predetermined target temperature. The sequence controller then determines a phase angle or a wavenumber that corresponds to a resultant power difference, and controllably turns on or of f the switching element based on its phase condition or wavenumber condition.
The AC power source that supplies electric power to heaters has a wide-ranging power-supply voltage of, for example, from 85V to 140V or from 187V to 264V. Therefore, when all heaters are supplied with electric power, about 2.7 times as large a power difference arises in a case in which the power-supply voltage is within a range of 85V to 140V, and about twice as large a power difference arises in a case in which the power-supply voltage is within a range of 187V to 264V. Additionally, the sequence controller controls an electric current supplied to the heaters so as to reach a predetermined temperature. Therefore, if thick paper is fed to the thermal fixing device, quantitatively greater electric power, i.e., greater electric current is supplied than in regular paper. Since the sequence controller controls the thermal fixing device so as to keep the thermal fixing device at the predetermined temperature, there is the possibility that an excessive quantity of electric power will be supplied depending on paper types. Therefore, there is a need to control an applied current of the heater so as to be kept below a maximum suppliable current value while always detecting the current, in order not to supply an excessive electric current to the heater that is a constituent element of the thermal fixing device.
However, regardless of electric power of the electric current to the heater, the maximum suppliable current value always is calculated while detecting the current, consequently load of the sequence controller increases and thermal efficiency decreases.
Additionally, when the thermal fixing device includes two or more heaters, there is a case in which some of the heaters are not supplied with an electric current while sheets of paper are being fed, depending on the paper type of transfer paper. In this case, an applied current is smaller than in a case in which all heaters are supplied with an electric current. Therefore, a maximum suppliable current value determined from this current value is set to be greater than in that case in which all heaters are supplied with an electric current. Therefore, a conventional problem resides in the fact that an excessive electric current is supplied when all heaters are supplied with an electric current after having determined the maximum value to be greater.